Maybe because most of the third party don't support in game advertising, such as Admob, RevMob, etc. For example, Appmobi doesn't have this, except to advertise another developer's game, which doesn't actually make revenue. Cocoonjs requires a premium account to take part in their ad revenue program and Phonegap I don't think has any ad revenue program.
I have been searching for a solid answer regarding implementing ads across iOS and Android. The best answer I came up with that you won't have to pay is implementing a plugin with your ad network's code.
The way you would implement the plugin is different for whatever development platform you're using.
For you UGS2012, if you can manage to get the game running smooth via Phonegap then having a plugin of some sort that works with Phonegap and has implemented the ad code will probably be the best approach.